1. Field of the Invention
The present invention relates to a charging device for charging secondary batteries, and particularly to a charging device for suppressing an excessive discharge current from a battery pack having lithium batteries or the like (including lithium ion batteries) when the battery pack is initially mounted in the charging device.
2. Description of the Related Art
With the development of portable devices such as cordless power tools, high-capacity secondary batteries, including nickel hydrogen batteries and nickel cadmium batteries, are increasingly being used as the power source for such portable devices. Another type of high-capacity secondary battery that is being used more and more in portable devices is the lithium battery, which produces a higher voltage than the nickel hydrogen or nickel cadmium batteries.
The lithium battery has a nominal voltage about 2-3 times greater than the more widely used nickel hydrogen and nickel cadmium batteries. The lithium battery also has an energy density about three times that of nickel cadmium batteries, while remaining compact and lightweight. Moreover, lithium batteries can discharge in a relatively low-temperature environment, thereby obtaining a stable voltage within a broader temperature range.
Obviously, the greater the voltage required for a cordless power tool or the like, the greater the number of battery cells that must be accommodated in the battery pack of the secondary battery. For example, since the nominal voltage of a nickel hydrogen battery cell is 1.2 V, twelve such battery cells are required in power tools operating on a battery voltage of 14.4 V and twenty cells must be included in a battery pack for a power tool operating at 24 V. However, since lithium batteries have a higher nominal voltage of 3.6 V, only four such battery cells are required in a battery pack to produce 14.4 V.
Battery packs having different voltages are provided to meet the voltage requirements of various portable devices. As a result, universal charging devices capable of charging battery packs with differing numbers of battery cells have become commonplace. One such universal charger is disclosed in Japanese patent application publication No. 2004-187366. Further, while battery packs for lithium batteries require a charging method using constant current and constant voltage control, as shown in FIG. 5, battery packs with nickel hydrogen cells or nickel cadmium cells require a charging method using constant current control, such as that shown in FIG. 6. Accordingly, there is an increasing number of universal charging devices capable of charging various types of battery packs having different battery voltages and requiring different charge control.
In order to perform constant current control when charging a battery pack having nickel hydrogen and nickel cadmium batteries, the universal charging device described above must set the output voltage to a value corresponding to the battery pack having the largest number of battery cells that can be charged with the charging device.
In the charging device disclosed in Japanese patent application publication No. 2004-187366, a relay switch for turning the path of the charging current on and off is provided between the charging source and the battery pack being charged. When the relay switch is closed in this charging device to begin charging and a low-voltage battery pack having few battery cells is mounted in the charging device, the charging device will apply a high-output voltage corresponding to a high-voltage battery pack with a large number of battery cells to this battery pack having few battery cells, causing an excessively large inrush current to flow as soon as the relay switch is closed. Consequently, the relay switch provided along the charging current path suffers great damage. Further, the relay switch is turned off when a battery pack is not mounted in the charging device or when the mounted battery pack is determined to be fully charged and charging is ended, thereby interrupting supply of the charging source. However, since the charging current is controlled to supply the highest output voltage required for a battery pack having the largest number of battery cells that can be charged by the charging device, power consumption is high, even when the relay switch is turned off.
To resolve the problem of an inrush current flowing from the charging source to the battery pack described above, Japanese patent application publication No. 2004-187366 proposes a charging device having an output voltage setting means capable of setting one of a plurality of output voltages. With this construction, the output voltage setting means sets the output voltage of the charging source to a first output voltage greater than the battery voltage required for the battery pack being charged when the relay switch is on for charging, and sets the output voltage to a second output voltage smaller than the first output voltage when the relay switch is off.
However, the charging device described above does not sufficiently reduce power consumption because the power supply circuit of the charging device continuously outputs a constant voltage during the standby state in which a battery pack is not mounted in the charging device or when the mounted battery pack is no longer being charged.
Further, the charging device disclosed in Japanese patent application publication No. 2004-187366 simultaneously uses the same power supply as the first power supply circuit for supplying power to charge the battery pack and as the second power supply of a control circuit in a microcomputer and the like. Therefore, halting the supply of power from the first power supply circuit also halts the supply of power for the control circuit. Since it is necessary to always output a charge voltage, this charging device cannot sufficiently reduce power consumption.
By providing a separate power supply from the first power supply circuit as the second power supply for the control circuit of the microcomputer and the like, it is possible to halt output from the first power supply circuit when not charging a battery pack and to eliminate the need for a relay switch isolating output of the first power supply circuit from the battery pack, thereby preventing the problem of a large inrush current described above. However, simply interrupting output does not prevent a large discharge current from momentarily flowing from the batter pack to the smoothing capacitor in the first power supply circuit the instant the battery pack is mounted in the charging device. Since a large charging/discharging current can reduce the life of a lithium battery and otherwise reduce the battery's performance, it is necessary to consider adding a special circuit for preventing such excessive discharge.